The invention relates primarily to tube heat-exchangers of the aforesaid kind in which the medium intended to flow in the tubes is a liquid or optionally a medium which changes phase during a heat-exchange process, and in which the medium intended to flow around the outer surfaces of the tubes is a gas. The heat-exchanqer is particularly intended for use in industrial applications, particularly in corrosive environments. It is primarily intended for extracting heat from flue gases, e.g. heat from the flue gases of oil and coal fired power stations. Heat-exchangers intended for this purpose need to be robust and powerful. They are therefore preferably made of steel. When the heat-exchangers are to be used in corrosive environments, it is often necessary to coat the surfaces of the heat-exchanger with an impervious corrosion inhibitor, for example, an enamel, unless the heat-exchanger is constructed from a corrosion resistant material throughout. Consequently, the invention is particularly directed to tube heat-exchangers of the kind which incorporate batteries of heat-exchange fins and in which the fins are secured firmly by expanding the tubes, and which are made of steel and provided with impervious surface coatings of a damage-resistant substance, preferably enamel.
It is generally recognized that in the case of tube heat-exchangers in which liquid flows through the tubes and gas flows around the outer surfaces thereof, the gas transfers heat much less effectively than the liquid. Consequently, it is necessary to enlarge the outer surfaces of the tubes. The two most common ways of achieving this are:
(a) By providing helical flanges on the outside of the heat-exchanger tubes. The flanges are normally welded to the tubes, so as to eliminate the heat resistance at the juncture between flange and tube. In addition to rotational regenerative heat-exchangers for direct heat exchange between two gases, e.g., regenerative air heaters of the Ljunqstrom type, the most common type of heat-exchanger used industrially in conditions where an enlarged outer tube surface is required are those fitted with helically wound tubes, i.e. with helical fins along the tubes. Otherwise, tube heat-exchangers with smooth tubes are used. Since gas leakages readily occur in said rotating heat-exchangers, they have been replaced progressively with helical-tube type heat exchangers.
(b) By fitting batteries of flat surface-enlarging fins to the outer surfaces of the heat-exchanger tubes. The fins are often made to a standard design for several heat-exchanger tubes. These fin batteries are mostly used in apparatus intended for general ventilation (comfort) and similar purposes. Consequently, the tubes and fins of such heat-exchangers are given comparatively small dimensions and are also made of a soft material, such as cooper or aluminum. One commonly applied method of achieving good heat transfer between the tubes and the fins, i.e. good contact with high contact pressure at the junction therebetween, is to secure the fins to the tubes by expanding the tubes radially into engagement therewith. This can either be effected mechanically with the aid of a mandrel or a spherical body which is drawn through respective tubes, or hydraulically by pumping liquid under high pressure through the tubes. Both methods are based on expanding the tubes radiallY so that the material of the tube stretches beyond the elastic limit of the tube material, so as to obtain permanent deformation and a high contact pressure.
With regard to fin-batteries used with heat-transfer apparatus for general ventilation purposes and like purposes, it is relatively easy to secure the fins by expanding the tubes mechanically or hydraulically in the aforesaid manner. It will be appreciated that in the case of such apparatus, the tubes and fins have small dimensions and are made of soft materials, such as copper or aluminum. In addition, the fins are provided with resilient collars around the holes through which the heat-exchanger tubes pass. This facilitates expansion and ensures that a given contact pressure constantly prevails between the tubes and the fins. The collars also often serve as spacers between the fins.
Fin batteries of this kind, however, have not been utilized in the aforesaid industrial applications, despite the advantages to be gained over heat-exchangers equipped with helically wound tubes. These advantages include:
greater surface enlargement PA1 lower pressure drop PA1 more stable heat-exchanger body PA1 cheaper heat-exchanger.
Thus, the more robust tube-exchanger required in industrial applications has primarily incorporated helically wound tubes, or in some cases smooth tubes. These tube heat-exchangers are mostly made of steel. There are several reasons why fin batteries of the aforesaid construction have not come into use industrially. For example, a number of difficulties and problems arise when fin batteries are to be made of steel, and particularly when they are to be provided with protective surface coatings. These problems are primarily as follows:
(a) It is more difficult to expand radially heat-exchange tubes which are made of steel. In order to expand the steel tubes hydraulically, it is necessary to use pressures of around 1000 bars in the case of tube thicknesses normally required in such heat-exchangers.
(b) It is difficult, if not impossible, to provide the steel fins with resilient collars around the holes through which the tubes pass. Among other things, the collars tend to crack.
(c) When providing the heat-exchanger surfaces with a protective covering, e.g. an enamel covering, it is difficult to ensure that the covering will be fully impervious, which is necessary in order to provide satisfactory protection against corrosion. In order for the enamel surface to be fully impervious, the surfaces of the heat-exchanger prepared to receive the enamel coating must be perfectly smooth and devoid of all cracks and other cavities. These surfaces should also be free of readily dislodged surface materials, such as welding slag or weld beads for example, capable of being knocked-off or otherwise removed when desooting the heat-exchanger or handling the same for some other reason, the removal of such surface materials being liable to leave cavities in the enamelled surface. It is not feasible to use resilient collars around the fin holes through which the tubes pass, since gaps and cracks around the collars would impair the enamelled surface. Such gaps and cracks cause, inter alia, bubbles to form in the enamel, which subsequently rupture and form discontinuities in the enamel as a result thereof. Even if they do not rupture, they are liable to cause imperfect surface covering and as a result, corrosion damage. Neither will this construction enable the fins to be fitted securely enough. It will be appreciated that flexing of the resilient collars creates cracks in the enamel coating.